Cities have vanished before, under force of time and an inconstant earth

Only the wind inhabits the cliff dwellings at Mesa Verde in Colorado, birds and vines the pyramids of the Maya. Sand and silence have swallowed the clamors of frankincense traders and camels in the old desert center of Ubar. Troy was buried for centuries before it was uncovered. Parts of the Great Library of Alexandria, center of learning in the ancient world, might be sleeping with the fishes, off Egypt's coast. Nothing lasts forever.

"Cities rise and fall depending on what made them go in the first place," said Peirce Lewis, an expert on the history of New Orleans and an emeritus professor of geography at Pennsylvania State University.

Changes in climate can make a friendly place less welcoming. Catastrophes like volcanoes or giant earthquakes can kill a city quickly. Political or economic shifts can strand what was once a thriving metropolis in a slow death of irrelevance. After the Mississippi River flood of 1993, the residents of Valmeyer, Ill., voted to move their entire town two miles east to higher ground.

What will happen to New Orleans now, in the wake of floods and death and violence, is hard to know. But watching the city fill up like a bathtub, with half a million people forced to leave, it has been hard not to think of other places that have fallen to time and the inconstant earth.